What is the difference between shart and and rukn in prayer?

Bismillah.

A rukn is a fard action and a shart is the fard outside of the action, i.e. to be in wudu is a shart which is required throughout the action and rukn is fard action which one does and leaves, i.e. the ruku (or sujood), one does ruku and in the next rakah returns to it.

And Allah(s) knows best.